The subject matter disclosed herein relates to a system and method of acquiring three-dimensional coordinates of points on a surface of an object and in particular to a system and method of operating a laser tracker in conjunction with a scanner device to track the position and orientation of the scanner device during operation.
The acquisition of three-dimensional coordinates of an object or an environment may be known using a variety of measurement devices. An environment is defined to be the collection of object surfaces within range of a measurement device. One type of measurement device is a time-of-flight system such as a laser tracker that directs a beam of light such as a laser beam toward a retroreflector target to be measured. In an embodiment, an absolute distance meter is used to determine the distance from the tracker to the retroreflector target based on a length of time it takes the light to travel to the target and return. By placing the retroreflector target in contact with an object surface, the coordinates of the object surface may be determined. A laser tracker directs the beam of light in a selected direction by rotating two axes with motors. The angles of rotation of the two axes are measured with angular transducers such as angular encoders. By measuring the one distance and two angles, a laser tracker may determine the three-dimensional (3D) coordinates of the retroreflector target. Some laser trackers have additional capability of measuring orientation of a target, thereby providing measurements of six degrees-of-freedom (6DOF).
An alternative to a time-of-flight measurement device is a scanning system that determines 3D coordinates based on triangulation principles. System such as laser trackers that make use of time-of-flight distance meters in some cases are more accurate than triangulation scanners, but triangulation scanners may be faster because they project a plurality of light spots onto the surface at each instant in time.
A triangulation scanner projects either a line of light (e.g. light from a laser line probe) or a pattern of light over an area (e.g. structured light) onto the surface. In the system, a camera is coupled to a projector in a fixed mechanical relationship. The projected pattern of light emitted from the projector is reflected off of the surface and imaged by the camera. Since the camera and projector are arranged in a fixed relationship, the distance and angles to the object may be determined from the projected pattern, the captured camera images and a baseline distance separating the projector and the camera according to trigonometric principles. Triangulation systems provide advantages in quickly acquiring 3D coordinate data over large areas.
In some systems, during the scanning process, the scanner acquires a series of 3D images, which may be registered relative to each other so that the position and orientation of each 3D image relative to the other 3D images is known. If the scanner is stationary, such registration is not necessarily. Similarly if the scanner is attached to a mechanical device having the ability to measure the position and orientation of the scanner, it is not necessary to provide such registration. Examples of such mechanical devices include articulated arm CMMs and Cartesian CMMs. Where the scanner is handheld and hence movable, various techniques may be used to register the images. One common technique uses features in the images to match overlapping areas of adjacent image frames. This technique works well when the object being measured has many features relative to the field of view of the scanner. However, if the object contains a relatively large flat or curved surface, the images may not properly register relative to each other.
Accordingly, while existing coordinate measurement devices are suitable for their intended purposes, the need for improvement remains, particularly in improving the registration of images acquired by a scanner device.